For those of iPhone 4S users who have a passcode enabled, Siri can still be activated from your lockscreen. So Siri wold let anyone bypass your iPhone Lockscreen to make a call, send a message, update Facebook, update Twitter or do more things without you.

Weng Chin shows us how this can be done on iPhone 4S below:
The feature to disable Siri feature at the lock screen from the exchange interface is not available now, but you can fix this problem by opening the settings application, go to the passcode lock preferences, finally toggle Siri to “Off”.